For optical recording to viably compete with magnetic recording, optical heads of reduced size, weight and cost must be developed while maintaining the same level of performance. The evolution of optical heads has produced light weight bulk optical systems, and, more recently, prototypes of complete and partial integrated optical systems. Integrated optical components have the advantages of being planar, having optical component alignment as part of fabrication, and facilitating integration with detectors and preamplifiers when using silicon based substrates.
A problem encountered in designing new optical-recording error-signal-detection methods and systems is observation of decollimation in the beam returning from the optical disk when the disk is out of focus for the objective lens, defocus, and asymmetry of the intensity in response to a tracking error. Generally, the methods for detecting the tracking error can be much simpler than for the focus-error detection, not requiring the critical alignment of optical components. The processing of the return beam should produce error signals which provide the resolution needed for adjustment of the objective lens position at levels much finer than its depth of focus and track width. A standard practice for correcting the position error for the objective focus is using a bipolar signal providing directional and amplitude information for the servo system. The method for correcting the tracking error is the use of a sinusoidal signal also providing an amplitude and directional information. The detection method for focus error heretofore used in waveguide systems has been the pupil obscuration method as described by A. Marchant, Optical Recording: A Technical Overview, Addison-Wesley, pp. 165-187 (1990).
New methods for processing optical recording information using waveguides are continually evolving. A reference which provides background for waveguides and waveguide gratings is Integrated Optics, Springer-Verlag (1975), edited by T. Tamir. Waveguide gratings are used to couple light into the waveguide and then to process information using a second lensing element or to couple and focus using focusing grating couplers. These concepts have been applied to magneto-optical (MO) systems as disclosed by Ura et al., An Integrated-Optic Disk Pickup Device, J. Lightwave Technol., LT-4, p. 913-918 (1986). These concepts have also been applied to write-once systems as disclosed in U.S. Pat. No. 4,833,561 and Japanese Patent Application, No. 62-166332. It is therefore desirable to have a simple, compact device for processing information.